A. Field of the Invention
This invention relates generally to steering light beams by the principle of the refraction of light. More particularly the invention relates to a light beam steering system, which in its simplest form consists of an adjustable optical wedge containing a plano-convex and a plano-concave lens element with nearly matching curvatures on the spherical surfaces. The spherical surfaces of the lens elements being disposed adjacent to one another with a means for maintaining a constant and uniform gap between them during specified modes of operation as a light beam scanner.
B. Description of Related Art
Most commonly, light beams are deflected or steered using mirrors where the principle of reflection is employed. Mirrors are ideal for this purpose when large angles of deflection are required, 45° or more. However, there are many applications that require only small deflections of an optical beam from a few arc seconds to a few degrees. The use of mirrors (as shown in FIG. 1) in these applications is cumbersome at best and often impractical since at least three mirrors are required to effect beam steering in such small angular ranges when the light beam's original optic axis before deflection must be maintained.
Deflected beam angles ranging from essentially 0° to ˜30° can readily be obtained by a rotating mirror 2 shown in FIG. 1. However, in order to accomplish this an incoming beam 4 must be diverted from the original direction by mirrors 6 and 8 resulting in the use of three mirrors in order to obtain a small deflection angle ?.
An alternate method for obtaining small angular deflections of a light beam is to refract the beam using an adjustable optical wedge. The principle of refractive beam steering is shown in FIG. 2. An incident laser beam 10 enters an optical wedge 12 at angle ?1. The wedge angle of the optic is θW. The angle δ is the deviation angle the transmitted light beam 4 makes with an incident direction 14.
Adjustable optical wedges date back to at least 1929 (H. Cox, U.S. Pat. No. 1,735,108, Nov. 12, 1929), but recent improvements in these devices have renewed interest in them for a variety of beam steering applications. One method for forming an optical wedge, that has been previously disclosed, is to use a liquid filled variable angle prism. Various methods for doing this in one form or another have been disclosed by several individuals and have resulted in U.S. Pat. Nos. 3,212,420, 3,337,287, 3,514,192 and others. The basic idea is to use two flat plates and an optical fluid that is nominally transparent to the incident light beam to form an adjustable optical wedge as shown in FIG. 3.
Referring to FIG. 3, a volume 16, defined by flat glass plates 18, 20 and bellows 22, is typically filled with transparent optical fluid that has a refractive index matching the refractive index of the flat plates 18, 20. The flat plates 18, 20 are adjusted in angle about a hinge 28 in order to obtain a wedge angle ? giving a desired beam deflection d to a transmitted beam 24 with respect to an initial beam 26. It has been pointed out that pressure and temperature effects in the optical fluid in volume 16 can lead to a variation of the index of refraction throughout the fluid volume thereby affecting both beam quality and deviation angle.
Another technique for forming an adjustable optical wedge has been disclosed by Swain et al. in U.S. Pat. No. 4,961,627. The apparatus and method disclosed by Swain is described in part in reference to FIG. 4. Some means must be provided for maintaining a gap between the spherical surfaces; otherwise they will come in contact with each other during the rotation of one surface within the other when forming the desired wedge angle causing damage to the optical surfaces.
Swain disclosed a lens unit 30 with an optical element 32 having a convex semi-spherical surface and another optical element 34 having a concave semi-spherical surface that are disposed with the semi-spherical surfaces in close proximity to each other. They remain separated by a gap 36 that is maintained by ball bearings 38. The gap 36 may be filled with an optical fluid. These ball bearings perform the necessary function of keeping a constant gap spacing at all times. Of course, means (not shown here) for supporting the optical elements and for adjusting a wedge angle d between optical elements 32 and 34, must also be provided. An incident light beam 40 is transmitted through the lens unit 30 and is deviated at a desired angle ? with respect to the optic axis 42.
There are some issues that arise when employing Swain's method for forming an optical wedge. First, it would appear that this technique might only be practical, due to cost, in steering applications that are more or less unique one-of-a-kind applications. It is believed that the perceived cost of this device may preclude its general wide spread acceptance in most laboratories. Second, it is noted that the bearings are disposed to move along the surface of the optical elements posing a risk of damage to the surface of these elements.
Third, it is noted that the ball bearings move along the spherical surfaces as one of the lens elements is rotated. Therefore, unless the curvatures are properly matched, a change in the gap uniformity will be induced, as the center of curvatures of the two semi-spherical surfaces shift with respect to each other during operation. If this is prevented from happening then the bearings will slide rather than roll along the surfaces. If an index matching fluid is used in the gap then this effect presents no difficulty. However, in some applications index matching fluid is not available and in these instances refractive errors which cannot readily be corrected if proper alignment is not maintained between the stationary and movable lens elements may be imparted to the transmitted beam.
The use of bearings to maintain alignment between the movable and stationary lens elements requires that the bearings alone provide this alignment. The method of actuation disclosed by Swain could lead to a conflict between the bearings and the actuators regarding this alignment.
On the other hand, if neither bearings nor gimbals are used to maintain gap spacing then the mechanical means of support and alignment become even more complex with added attendant cost. There have been innovations involving rather complex mechanical systems that do not utilize gimbals to provide for motion of one lens with respect to the other which claim rotation along the spherical surfaces in a stable and uniform manner, e.g. Lecuyer and Quinn who disclose a Retrofit Line of Sight Stabilization Apparatus and Method, U.S. Pat. No. 5,424,872; and Momochi who discloses an Apex-angle Variable Prism and Video Camera, U.S. Pat. No. 6,157,405. However, this problem has been solved by another method for providing a uniform gap spacing that requires neither gimbals nor complex and expensive mechanical systems.
This method for forming an adjustable optical wedge has been disclosed by Dube' in U.S. Pat. No. 6,320,705, the contents of which are hereby incorporated by reference, and is shown in partial cross-section in FIG. 5. This apparatus is called a “Lubricated Adjustable Optical Wedge” (LAOW), (also known as a “matched-lens” beam steerer). The optical wedge in this case consists of a lens unit 44 containing a plano-concave lens element 46 and a plano-convex lens element 48. Spherical surfaces 50 and 52 of these two lens elements 46, 48 are disposed in lubricated contact with each other with a gap 54 remaining between them. The spherical surfaces 50, 52 have radii of curvature that are nominally the same. The gap 54 between the lens elements is filled with an optical fluid or lubricant, which wets both spherical surfaces 50, 52 leaving essentially no voids or air pockets within the gap 54. The term “optical fluid” for purposes of this disclosure is taken to mean a fluid that is nominally transparent to an incident light beam and that has lubricating properties.
One of the lens elements (in this case lens element 46) will be fixed securely to a base of support (not shown) and is regarded as a stationary element. The other lens element 48 will be operated or subject to movement by applying an external force to displace or rotate it about the center of curvature causing the formation of a wedge angle ?. A light beam 56 enters the lens element 48 through a surface 58 and is transmitted through the lens unit 44 and exits a surface 60 as shown making an angle d with an optic axis 62. The forces of capillary action and surface tension acting within the gap 54 can be the sole means for holding the lens elements 46, 48 together. The optical fluid within the gap 54 also keeps the spherical surfaces 50, 52 from contacting each other, thereby preventing damage to these polished optical surfaces during operation. The rotated element 48 will maintain its spherical surface 52 in intimate contact with the lubricant interface during this movement. The forces of capillary action and surface tension are sufficient in some modes of operation to maintain a fixed and uniform gap between the two lens elements 46, 48 and at the same time hold the lens elements 46, 48 together without any other means of support, thus avoiding the use of expensive mechanical mounts or gimbaled systems.
A most important area where this innovation functions well is in applications where adjustments of the wedge angle are infrequent and normally accomplished by manually operated actuator/driver systems. Devices using the LAOW concept have been manufactured and are currently on the market.
The LAOW adjustable optical wedge greatly simplifies the mechanisms needed to actually displace or rotate one of the lens elements 46, 48 within the spherical cavity of the other in order to create a wedge angle. This was aided by another innovation disclosed in the Dube' Patent, i.e. a lens ring with a spherical surface was used into which the movable lens is mounted. The outer surface of this ring has a spherical contour that provides a simple means for smooth, precise, and repeatable positioning of the movable lens.
Exacting measurements of the performance of a high precision LAOW device have resulted in a repeatability of ±0.1 arc seconds in the angle of deflection. These measurements were actually limited by the resolution of the instruments used to conduct these tests. Contrary to the innovations disclosed by Swain et. al., Lecuyer and Quinn, and Momochi, the method taught by Dube' may be used in a practical sense with small lens diameters as well as large lenses, which may range from a few millimeters up to hundreds of millimeters without any change in the driving mechanisms or addition of mechanical support mechanisms except for scaling the size appropriately.
The LAOW concept has been studied extensively and undergone refinements in the methods of actuation that have led to much smaller and simpler devices. One area that remained to be investigated was the operational characteristics of a LAOW device in a continuous scanning mode. A study of the concept under a continuous duty scan mode was conducted under a contract with the Eglin Air Force Research Laboratory. In this study lens units ranging from 2″ to 4″ in diameter and cycle rates up to ˜9 radians/second were used in the tests.
During the course of the Air Force studies it was found that 1) the lens units would indeed stay together up to the maximum cycle rate of 9 Hz (no lens units separated during these tests), 2) settling occurred when the lens unit was left in a stationary mode for periods of time ranging for a couple hours to several days, and 3) the uniformity of the lubricated gap would change over a period of time while scanning. The settling resulted in the gap narrowing and, in some cases, glass to glass contact was made, which led to scratches in the lenses upon resumption of scanning. Changes in the uniformity of the lubricated gap directly affects the optical wedge angle and the pointing direction of the transmitted light beam. A device that had been calibrated for high precision beam steering could thus lose its calibration if the uniformity of the lubricated gap were to change during the course of operation.
Although these phenomena had not been observed prior to the Eglin test program, it was not unreasonable to suspect such behavior. Therefore, another innovative technique for forming the optical wedge was included in the original proposal to Eglin that was tested and in fact was reduced to practice during the course of the Eglin contract. This innovation is, in effect, a method for forming an adjustable optical wedge that maintains a uniform gap between two lens elements both while in storage and during operation in a continuous scanning mode. As with the Dube' Patent, this new innovation requires neither complicated mechanical support structure to maintain alignment between lens elements nor gimbals.
It is important to maintain a constant spacing in the gap and uniformity in the gap volume in order for the beam steering unit to impart precise and repeatable angular deflections to the transmitted beam. The method taught by Swain utilizes ball bearings to provide this essential feature. The method taught by Lecuyer and that taught by Momochi use complicated mechanical support means. The method taught by Dube' relies solely on the forces of surface tension and capillary action to provide this characteristic. All of these innovations utilize lens elements comprising the optical wedge that are of a similar nature. The distinguishing feature amongst them is the technique employed to maintain constant and uniform gap spacing during the formation of the wedge angle.
The Dube' approach was seen to work well under certain conditions as discussed earlier. However, the perceived areas of applicability for refractive beam steering may subject the lens unit to long storage periods, continuous scanning at high rates for long periods, and deployment in platforms subject to vibrations and environmental contaminants. Under these conditions the Dube' innovation cannot be relied upon to maintain a uniform and constant gap spacing. The present innovation does not rely on the techniques described in these prior arts to provide this important characteristic.